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The five biggest unanswered questions in physics right now LynetteCook/Science PhotoLibrary

What is the nature of the dark ? Just over 95% of our universe comes in the shrouded form of dark energy and matter that we can neither explain nor directly detect. Catherine Heymans explores this enigma and describes where we will look next in our search for darkness

This year the Planck space mission released exqui- Dark matter is exactly what it says on the tin: it is Catherine Heymans site observations of the early universe, providing dark and comprised of a mysterious substance that is a reader in the strongest evidence yet that the universe we live does not emit or absorb light. We only know it exists astrophysics at the in is very dark indeed. Its precise results show that because of its gravitational effects on the normal mat- University of our universe is composed of 26.8% dark matter and ter that we can see. Dark energy is less well described Edinburgh, UK, and a member of the Young 68.3% dark energy, while less than 5% is made up of by its label, being an invisible source of energy that Academy of the stuff we are familiar with on . With their drives the post-Big-Bang expansion of the universe Scotland, e-mail long-standing quest to make these precision meas- to mysteriously accelerate. Together, these two dark [email protected] urements essentially now concluded, cosmologists entities play out a cosmic battle of epic proportions. are rapidly turning their attention to a much big- While the gravity of dark matter slowly pulls struc- ger and further-reaching question: what is the exact tures in the universe together, dark energy fuels the nature of this dark universe? universe’s accelerating expansion, making it ever

Physics World October 2013 33 Physics World at 25: Questions physicsworld.com

will be caught crashing into some of the xenon par- ticles. How successful this new experiment will be in its quest to uncover the nature of dark matter will depend on just how much of a wimp the dark-matter particle turns out to be. An unquestionable direct detection of a dark-matter particle would be one of the most significant discoveries of this century, finally confirming Fritz Zwicky’s theory, which was ridiculed when he proposed it in 1933.

Exposing dark energy While the astronomical community is now fairly united in postulating the existence of an invisible dark-matter particle, the same cannot be said about its support for the simplest explanation for dark energy. Observations that the expansion of our uni- verse is accelerating are most easily explained by con-

Van Waerbeke,Van Heymans, Canada–France–Hawaii Telescope Lensing Survey sidering the extra energy associated with the vacuum that permeates the universe. According to quantum Dark cosmic web The distortion of light from about 10 million theory, empty space is filled with a swarm of virtual galaxies, which is bent as it passes clumps of dark matter, has shown particles with a wide range of masses that can briefly that dark matter in the universe is distributed as a network of gigantic pop in and out of existence. As mass and energy are dense regions (white), separated by a vast emptiness. equivalent, the growing vacuum within an expanding universe acts like a bank of unlimited energy, inflat- harder for those dark-matter structures to grow. ing the whole universe at an accelerated speed. It is widely believed that to truly understand the Unfortunately, there is a problem with this simple dark universe, we will need to invoke some new and elegant vacuum solution to the nature of dark physics that will forever change our cosmic view. energy. Particle physicists can make a theoretical As the conclusion of this dark quest could be so far estimate for the energy of a vacuum and they find reaching, astronomers are approaching the task with that it is 120 orders of magnitude larger than the care, using a series of independent and meticulous dark energy that the Planck results show. This wild observations. Efforts include the Canada–France– discrepancy has opened up a wide range of exciting Hawaii Telescope Lensing Survey, which has directly new dark-energy theories including exotic models mapped out the invisible cosmic web of dark mat- such as a multiverse that resembles the middle of ter by observing how its mass bends space and time, an Aero chocolate bar. Perhaps our universe is one lensing the light of very distant galaxies. Projects Aero bubble being pulled by our neighbouring Aero- such as the Sloan Digital Sky Survey are accurately bubble universe? charting the locations of billions of galaxies, which Many cosmologists believe that the dark-energy closely trace the distribution of dark matter because phenomenon indicates that we need to look beyond this gravitationally attractive substance dictates Einstein’s theory of general relativity. By observ- where and when galaxies form. Galaxies also carry ing how dark-matter structures change over cosmic with them a signal imprinted in the distribution of time, we can investigate how dark energy evolves and normal matter just after the Big Bang that can be test gravity for the first time on cosmological scales. seen in how galaxies cluster in the cosmos today. Just as Einstein revolutionized our understanding of Newtonian gravity, confirmed through observations Capturing dark matter of the , so new observations of gravity on Astronomers have put their theories of dark mat- cosmological scales may bring about another revolu- ter to the test, finding that a very wide variety of tion in our understanding of gravity. observations all agree with a single theory, termed Two major new international projects will lead the “concordant cosmology”. This overwhelming our quest to discover what the dark-matter particle body of evidence supports the theory that dark mat- is and why the expansion of our universe is appar- ter is made up of weakly interacting matter particles ently accelerating. The Euclid satellite, due to (WIMPs), and the challenge is now on for particle launch in 2020, will image the full dark sky from To truly physicists to go out and catch or create one. above the Earth, while the Large Synoptic Survey Several attempts have already been made to trap Telescope, due to see first light in 2019, will image understand the a dark-matter particle, but any hints of success have the full Southern sky from a mountain top in Chile. dark universe, so far been controversial and open to interpreta- Both of these projects will chart the distant universe we will need to tion. The next major leap in the search for a fleeting with exquisite precision, utilizing a diverse range of invoke some glimpse of a dark-matter particle in flight is taking cosmological tools to map out the evolution of dark- new physics shape not in space but nearly 1.5 km under the Black matter structures and document the expansion and that will forever Hills of South Dakota. The LUX-ZEPLIN experi- curvature of space and time from 10 billion years ago ment will use nine tonnes of liquid xenon as its dark to the present day. Exciting times are ahead for our change our butterfly net. The hope is that a few of the trillions understanding of the fundamental physics that gov- cosmic view of WIMPs that pass through the Earth every second ern the dark side of the universe.

34 Physics World October 2013